A. Shirakawa

The KEKB injector linac

Abstract An 8-GeV electron/3.5-GeV positron injector for KEKB was completed in 1998 by upgrading the existing 2.5-GeV electron/positron linac. The main goals were to upgrade its accelerating energy from 2.5 to 8 GeV and to increase the positron intensity by about 20 times. This article describes not only the composition and features of the upgraded linac, but also how these goals were achieved, by focusing on an optics design and commissioning issues concerning especially high-intensity single-bunch acceleration to produce positron beams.

Intense positron beam at KEK

Abstract A positron beam is a useful probe for investigating the electronic states in solids, especially concerning the surface states. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diffraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 10 6 e + /s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity. One of the results using a high intensity pulsed positron beam is presented as a study of the origins of a Ps emitted from SiO 2 . We also describe the two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurement system with slow-positron beams and a positron microscope.

Present Status of the Slow Positron Facility at KEK

There has been increasing interest in the use of slow positron beams in various fields of science. However, because of its poor intensity a radioactive-isotope-based positron beam is not so versatile as a laboratory-based electron beam. To obtain intense beams accelerator-based positron sources[1,2,3] and reactor-based ones[4] have been developed and used successfully in studies of various materials. In this brief report a new slow positron facility at KEK is described. The facility consists of a dedicated 50 MeV linac, an assembly of slow positron generator, a slow positron transport line and an experimental station for positronium time-of-flight (Ps-TOF) spectroscopy. Figure 1 shows an overview of the facility.

An overview of the slow-positron beam facility at the photon factory, KEK

The KEK slow-positron source is in the final stage of construction. The beam line comprises a 31 m long vacuum duct within an axial magnetic field and a following electrostatic guided section. In order to vary the energy of a positron beam dedicated to depth-profile measurements, a high voltage station capable of applying 60 kV has been installed in the beam transport system. The target assembly (a water-cooled tantalum rod of 5 radiation lengths and a moderator with multiple tungsten vanes) and the following straight section (8 m; used for positron storage) are under high voltage. The beam duct located downstream is at ground potential. Positron beams passing through this region have a high kinetic energy. A focusing triplet quadrupole lens and a moderator on the retarding electrode are located at the end of the magnetic transport. This beam line has 9 right-angle-curved ducts, comprising a radius of curvature of 40 cm. Positrons with a maximum energy of 60 keV are guided by bending magnets attached to the beam-transport ducts. A transport system to switch from magnetically guided to electrostatically guided has been installed. The design of the brightness-enhancement stage of the positron beam for positron re-emission microscopy is in progress. In a preliminary experiments at 2.0 GeV with a 2 kW primary beam, 4×106e+/s of slow positrons were observed by detecting annihilation γ-rays at the end of the magnetic beam-transport line. Further improvements are expected by careful surface and thermal treatments of the moderator.

KEK-IMSS Slow Positron Facility

The Slow Positron Facility at the Institute of Material Structure Science (IMSS) of High Energy Accelerator Research Organization (KEK) is a user dedicated facility with an energy tunable (0.1 – 35 keV) slow positron beam produced by a dedicated 55MeV linac. The present beam line branches have been used for the positronium time-of-flight (Ps-TOF) measurements, the transmission positron microscope (TPM) and the photo-detachment of Ps negative ions (Ps−). During the year 2010, a reflection high-energy positron diffraction (RHEPD) measurement station is going to be installed. The slow positron generator (converter/ moderator) system will be modified to get a higher slow positron intensity, and a new user-friendly beam line power-supply control and vacuum monitoring system is being developed. Another plan for this year is the transfer of a 22Na-based slow positron beam from RIKEN. This machine will be used for the continuous slow positron beam applications and for the orientation training of those who are interested in beginning researches with a slow positron beam.

Progress of 7-GeV SuperKEKB Injector Linac Upgrade and Commissioning

KEK injector linac is being upgraded for the SuperKEKB project, which aims at a 40-fold increase in luminosity over the previous project KEKB. SuperKEKB asymmetric electron and positron collider with its extremely high luminosity requires a high current, low emittance and low energy spread injection beam from the injector. Electron beams will be generated by a new type of RF gun, that will inject a much higher beam current to correspond to a large stored beam current and a short lifetime in the storage ring. The positron source is another major challenge that enhances the positron bunch intensity from 1 to 4 nC by increasing the positron capture efficiency, and the positron beam emittance is reduced by introducing a damping ring, followed by the bunch compressor and energy compressor. The recent status of the upgrade and beam commissioning is reported.

Development of a position-sensitive gamma detector based on a scintillating fiber for a 2DACAR study

Abstract A position-sensitive gamma detector based on a scintillating fiber is described. At the KEK PF slow-positron facility, an intensity of more than 108 e+/s has already been achieved at the target point of the sample chamber utilizing the PF 2.5 GeV electron linac as its primary beam source, and studies for producing a positron microbeam have been carried out. In parallel with this, we have tried to realize a position-sensitive gamma detector for a positron microbeam. We attempted to use a fiber-optic scintillator for a position-sensitive detector. The glass compositions were alkaline earth, gadolinium, and terbium loaded silicate glasses. The glass was used as the core glass to fabricate bundles of fibers, with each fiber surrounded by cladding glass. Fiber sizes of 6 μm were used, which allowed an image resolution of greater than 20 line pairs per mm. The glass-fiber optic scintillating plate was coupled to an image intensifier. The image-intensifier output image was observed by a CCD camera.